The present invention relates to a ferrite oxide magnetic material used as a material for a core of an inductance device and particularly relates to a ferrite oxide magnetic material used as a material for a core of a resin-molded type chip inductor.
To meet the needs of small size, light weight, high accuracy, and so on, in the field of resin-molded type chip inductors, fixed coils, and so on, for which demands have become increased rapidly in recent years in the field of television sets, video recorders, mobile communication machines, and so on, there has been an increased demand for narrow tolerance and high reliability on these parts. To meet the demand, a ferrite core material used as a material for magnetic cores of these parts is required to have the following characteristic.
(1) The material has a low initial magnetic permeability (.mu.i) for fine adjustment of inductance due to winding. Particularly in the case of a vertically-placed type ferrite core, the initial magnetic permeability needs to be low (.mu.i: not higher than 8) so that the fine adjustment can be made by a half turn.
(2) The material has a high sintered density.
(3) The material has a characteristic in which the change of inductance is small against external stress due to a molded resin. The ratio of the change of inductance against external stress, that is, the ratio of the change of inductance against a load applied to a rod-like sample so as to be parallel with a direction of magnetization is hereinafter referred to as stress-resisting characteristic. That is, the material has an excellent stress-resisting characteristic.
(4) The material has a characteristic in which the change of inductance is small against both external stress due to a molded resin and external magnetic field applied to the material. The ratio of the change of inductance against both external stress and external magnetic field, that is, the ratio of the change of inductance against a load applied to a rod-like sample in parallel with a direction of magnetization and an external magnetic field which are applied and released is hereinafter referred to as stress-resisting and magnetic-field-resisting characteristic. That is, the material has excellent stress-resisting and magnetic-field-resisting characteristic.
When a molded inductor device was assembled, the device might be exposed to a magnetic field of high magnetic flux density. Accordingly, particularly in the stress-resisting and magnetic-field-resisting characteristic, there was an important question whether initial characteristic could be kept or not when the device was actually put into a final product even in the case where the device per se was adjusted to have a predetermined value of inductance. Further, even after the device was actually put into a final product, the chip inductor per se was not shielded magnetically. Accordingly, it was also necessary to avoid the bad influence of magnetization generated in electromagnetic parts around the chip inductor.
As an attempt to meet the aforementioned demand, there is known an attempt to improve stress-resisting characteristic by use of ferrite containing basic composition described in JP-A-2-60110 and JP-A-2-137301. However, sufficiently high sintered density to be practically satisfactory cannot be achieved when a core material having a low initial magnetic permeability .mu.i of not higher than 8 is to be obtained by use of the ferrite material containing such basic composition. Reduction of sintered density causes a problem in lowering of stress-resisting characteristic, penetration of a resin or an adhesive agent into pores of a ferrite core, and so on. Accordingly, sintered density cannot be reduced. On the contrary, when sintered density is to be increased, the initial magnetic permeability is increased to be not smaller than 8. In any case, a core material having practically satisfactory characteristic can be hardly obtained.
JP-A-8-51011 and JP-A-8-51012 disclose: an oxide magnetic material containing from 0 to 10 wt % of SiO.sub.2 (exclusive of 0 wt %), and from 0 to 10 wt % of Bi.sub.2 O.sub.3 (exclusive of 0 wt %) in addition to main components consisting of from 5 to 24 mol % of Fe.sub.2 O.sub.3, from 0 to 30 mol % of CuO (exclusive of 0 mol %), and aresidual part of NiO; an oxide magnetic material containing from 0 to 0.5 wt % of Co.sub.3 O.sub.4 (exclusive of 0 wt %) in addition to the main components; an oxide magnetic material containing from 5 to 24 mol % of Fe.sub.2 O.sub.3, from 0 to 30 mol % of ZnO (exclusive of 0 mol %), from 0 to 30 mol % of CuO (exclusive of 0 mol %), and a residual part of NiO; and an oxide magnetic material containing from 0 to 0.5 wt % of Co.sub.3 O.sub.4 (exclusive of 0 wt %), from 0 to 10 wt % of SiO.sub.2 (exclusive of 0 wt %), and from 0 to 10 wt % of Bi.sub.2 O.sub.3 (exclusive of 0 wt %) in addition to the main components in order to reduce both the change of inductance due to external stress and the lowering of inductance after application of an external magnetic field.
However, the composition discussed by way of examples in JP-A-8-51011 and JP-A-8-51012 is Fe.sub.2 O.sub.3 : 5, 10, 20 and 24.5 mol %; and ZnO: 7 and 30 mol % (other constituent elements are omitted). That is, no composition but this composition is not discussed in JP-A-8-51011 and JP-A-8-51012. Further, the stress-resisting and magnetic-field-resisting characteristic of a core obtained in the range of this composition is insufficient. Accordingly, to meet the demand for both high accuracy and high reliability, greater improvement of performance as to total characteristic including initial magnetic permeability, sintered density, magnetic field-resisting characteristic, stress-resisting characteristic, stress-resisting and magnetic-field-resisting characteristic, etc. has been required.
For example, a background-art ferrite core material containing the aforementioned basic composition, that is, containing, as basic composition:
Fe.sub.2 O.sub.3 : 48 mol %; PA1 NiO: 49 mol %; and PA1 CuO: 3 mol %; PA1 Pbo: 6 wt %; and PA1 talc: 6.5 wt %; PA1 the ferrite oxide magnetic material further containing, as components subsidiary to the basic composition: 0.01 to 15 wt % of lead oxide calculated in terms of PbO; and 0.01 to 15 wt % of silicon oxide and/or talc calculated in terms of SiO.sub.2 ; PA1 the ferrite oxide magnetic material having an initial magnetic permeability of not higher than 8, a sintered density of not lower than 4.8 g/cm.sup.3 and a stress-resisting and magnetic-field-resisting characteristic in a range of .+-.5% calculated in terms of the rate .DELTA.L/L of the change of inductance due to the condition of a magnetic field of 1000 G under a compressive stress P=5 (kg/mm.sup.2) parallel with a direction of magnetization.
and containing as subsidiary components in addition to the basic composition:
exhibits the following characteristic: a sintered density of 4.6 g/cm.sup.3 ; an initial magnetic permeability .mu.i of 6.0; and stress-resisting characteristic .DELTA.L/L(%) of -9% (at 1 ton/cm.sup.2). If an initial magnetic permeability .mu.i of about 6 is to be obtained in such a background-art ferrite core, not only the sintered density becomes low to be 4.6 g/cm.sup.3 but also the stress-resisting characteristic becomes large to be -9% (at 1 ton/cm.sup.2).
Further, FIG. 8 shows the relation between the amount of PbO--SiO.sub.2 glass and the initial magnetic permeability .mu.i in the case where the amount of PbO--SiO.sub.2 glass in the ferrite having the aforementioned composition is changed. It is apparent from FIG. 8 that the initial magnetic permeability of the ferrite having the composition is little reduced even in the case where the amount of PbO--SiO.sub.2 glass is increased.
Furthermore, JP-A-3-91209 discloses a chip inductor characterized in that a drum type core formed from a ferrite composition which is a spinel type composition containing 25 to 45 mol % of Fe2O3, 0 to 20 mol % of ZnO and residual part of NiO and CuO with the molarity of NiO being larger than the molarity of CuO and further containing 0.1 to 12 wt % of Bi2O3, and 0.05 to 4.0 wt % of SiO.sub.2 as trace components is wound with a coil and molded with an epoxy resin. In JP-A-3-91209, there is a further description that inductance is reduced when a stress is applied to the chip inductor by the epoxy resin and an external magnetic field is applied to this chip inductor.